The present invention relates to an improved cap for use with containers whereby removal of the cap leaves behind a telltale ring member. There are presently available many pilfer-proof closure or cap constructions of this type. Generally, they are constructed with a threaded skirt portion extending downwardly from the top wall of the cap and a pilfer-proof ring member attached to the lower end of the skirt. The ring is attached by frangible members which break upon unthreading of the cap from the container due to engagement of the ring member below a protruding ledge or shoulder on the container.
With metal closures, the formation of the pilfer-proof ring is typically effected after the closure has been placed onto the container. More particularly, the pilfer-proof ring is secured under a cooperating shoulder on the container neck by a rolling operation. With the advent of molded plastic caps, the pilfer-proof ring portion of the cap is performed as part of the initial molding of the cap. This presents some problems with respect to structuring the pilfer-proof ring and frangible members so that the cap can be threaded onto the container without breaking the frangible members. If breakage occurs, the ring is obviously of no value for use as an indicator of tampering with or removal of the closure.
The molding of plastic caps with pilfer-proof rings also presents some problems. The molding operation requires an internal die member for shaping the internal surface of the cap. This die member has external threads just like the bottle with which the cap is to be used. This thread structure and other portions of the die required for forming the pilfer-proof ring can interfere with removal of the cap by obstructing the ring and causing it to break away just as if the cap were being unthreaded from the container.
Attempts have been made to design plastic caps to overcome the problems associated with molding and applying the cap to the container. My prior patent, U.S. Pat. No. 4,322,012 discloses a molded plastic pilfer-proof cap in which the pilfer-proof ring is attached to the skirt by frangible members and constructed internally with locking members having the appropriate camming surface to permit threading of the cap to the container without breaking of the frangible members. Molding of this type of cap construction, however, typically requires a collapsible inner die member.
U.S. Pat. No. 4,147,268 also discloses a molded plastic pilfer-proof cap. In this cap the pilfer-proof ring is constructed with internally protruding locking members which extend at an angle so as to permit the cap to be removed from the internal die member by unthreading. This same structure permits the cap to be threaded onto the container. In addition, the locking members are provided with camming surfaces to permit sliding over the locking shoulder of the container as the cap is fully threaded onto the container.